It is unclear whether alkylating agents induce apoptosis because they damage DNA, or because they damage other cellular targets. Isogenic Chinese hamster ovary (CHO) cell lines varying in the repair of O6-alkylguanine (O6AlkG) were examined for their propensity to undergo alkylation-induced apoptosis. Robust O6AlkG repair virtually eliminated the apoptogenic effects of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU, carmustine), as did the expression of BCL-2. O6AlkG repair had no effect on apoptosis induced by tumor necrosis factor alpha or by gamma-irradiation. We conclude that alkylating agents induce apoptosis by virtue of their ability to modify DNA bases and, more specifically, that O6AlkG lesions can trigger such programmed cell death.